Posterior condylar offset and posterior tibial slope targets to optimize knee flexion after unicompartmental knee arthroplasty

Purpose To evaluate the relationship between posterior tibial slope (PTS), posterior condylar offset (PCO), femoral sagittal angle (FSA) on clinical outcomes, and propose optimal sagittal plane alignments for unicompartmental knee arthroplasty (UKA). Methods Prospectively collected data of 265 medial UKA was analysed. PTS, PCO, FSA were measured on preoperative and postoperative lateral radiographs. Clinical assessment was done at 6-month, 2-year and 10-year using Oxford Knee Score, Knee Society Knee and Function scores, Short Form-36, range of motion (ROM), fulfilment of satisfaction and expectations. Implant survivorship was noted at mean 15-year. Kendall rank correlation test evaluated correlations of sagittal parameters against clinical outcomes. Multivariable linear regression evaluated predictors of postoperative ROM. Effect plots and interaction plots were used to identify angles with the best outcomes. (p < 0.05) was the threshold for statistical significance. Results There were significant correlations between PTS, PCO and FSA. Younger age, lower BMI, implant type, greater preoperative flexion, steeper PTS and preservation of PCO were significant predictors of greater postoperative flexion. There were significant interaction effects between PTS and PCO. Effect plots demonstrate a PTS between 2 degrees to 8 degrees and restoration of PCO within 1.5 mm of native values are optimal for better postoperative flexion. Interaction plot reveals that it is preferable to reduce PCO by 1.0 mm when PTS is 2 degrees and restore PCO at 0 mm when PTS is 8 degrees. Conclusion UKA surgeons and future studies should be mindful of the relationship between PTS, PCO and FSA, and avoid considering them in isolation. When deciding on the method of balancing component gaps in UKA, surgeons should rely on the PTS. Decrease the posterior condylar cut when PTS is steep, and increase the posterior condylar cut when PTS is shallow. The acceptable range for PTS is between 2 degrees to 8 degrees and PCO should be restored to 1.5 mm of native values.